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A Microporous Metal−Organic Framework with Lewis Basic Nitrogen Sites for High C2H2 Storage and Significantly Enhanced C2H2/CO2 Separation at Ambient Conditions Hui-Min Wen,†,§ Huizhen Wang,‡,§ Bin Li,† Yuanjing Cui,‡ Hailong Wang,† Guodong Qian,*,‡ and Banglin Chen*,†,‡ †

Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249-0698, United States State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, School of Materials Science &Engineering, Zhejiang University, Hangzhou 310027, China

‡

S Supporting Information *

ABSTRACT: A novel metal−organic framework (MOF), [Cu2L(H2O)2]· 7DMF·4H2O [ZJU-40; H4L = 5,5′-(pyrazine-2,5-diyl)diisophthalic acid], with Lewis basic nitrogen sites has been constructed and structurally characterized. Owing to the combined features of high porosity, moderate pore sizes, and immobilized Lewis basic nitrogen sites, the activated ZJU-40a exhibits the second-highest gravimetric C2H2 uptake of 216 cm3 g−1 (at 298 K and 1 bar) among all of the reported MOFs so far. This value is not only much higher than that of the isoreticular NOTT-101a (184 cm3 g−1), but also superior to those of two very promising MOFs, known as HKUST-1 (201 cm3 g−1) and Co-MOF74 (197 cm3 g−1). Interestingly, the immobilized nitrogen sites in ZJU-40a have nearly no effect on the CO2 uptake, so ZJU-40a adsorbs a similar amount of CO2 (87 cm3 g−1) compared with NOTT-101a (84 cm3 g−1) at 298 K and 1 bar. As a result, ZJU-40a shows significantly enhanced adsorption selectivity for C2H2/CO2 separation (17−11.5) at ambient temperature compared to that of NOTT-101a (8−9), leading to a superior MOF material for highly selective C2H2/CO2 separation.

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INTRODUCTION Acetylene (C2H2) has been widely used for the synthesis of many chemical products and electric materials, such as polyurethane and polyester plastics.1 The high-purity C2H2 is highly in demand for the preparation of these important materials. Given the fact that carbon dioxide (CO2) commonly exists in many industrial processes, it is very important to separate C2H2 from CO2 impurities in order to obtain the highpurity C2H2. However, the C2H2/CO2 separation is a very challenging task because both gas molecules have similar molecular shapes (3.32 × 3.34 × 5.70 vs 3.18 × 3.33 × 5.36 Å) and boiling points (−84 vs −78.5 °C).2 Currently, the main technology for C2H2/CO2 separation is cryogenic distillation, which is very costly and energy-consuming. An alternative and energy-efficient strategy is to use porous materials for adsorptive separation technology. Among the diverse porous materials, metal−organic frameworks (MOFs) have much potential for this important industrial separation. Porous MOFs have been intensively exploited as adsorbents for gas storage and separation in the last 2 decades.3−5 Attributed to their high porosity, tunable pore/shape, and adjustable chemistry, quite a number of important and challenging separations, such as CO2 capture and separation, light hydrocarbon separation, O2/N2 separation, CO/N2 separation, etc., have been achieved by using new MOFs as © XXXX American Chemical Society

absorbents whose separation capacities are greatly superior to those of zeolite and activated carbon materials.6−10 Among the diverse gas separations, separation of the C2H2/CO2 mixture (50:50, v/v) still remains a great challenge and has not been fully developed so far.11 Nowadays, only a few MOF examples have been reported for this gas separation.12−16 To evaluate a porous material for C2H2/CO2 separation, both the adsorption selectivity and uptake capacity are the most important criteria. However, for the reported MOFs, it has been difficult to simultaneously achieve these two requirements until now. For example, the well-known HKUST-1, exhibiting high C2H2 storage capacity,17 has a relatively low C2H2/CO2 separation selectivity.15 Although UTSA-50 with both open metal sites (OMSs) and Lewis basic nitrogen sites has a slightly higher selectivity, its relatively low Brunauer−Emmett−Teller (BET) surface area of 604 m2 g−1 has limited its C2H2 uptake capacity.16 Therefore, it is still a grand challenge to target high C2H2 uptake and high adsorption selectivity simultaneously for a single MOF material. Special Issue: Metal-Organic Frameworks for Energy Applications Received: March 28, 2016

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DOI: 10.1021/acs.inorgchem.6b00748 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Single-crystal X-ray diffraction study indicated that ZJU-40 crystallizes in the hexagonal space group R3̅m. The framework of ZJU-40 is composed of paddlewheel dinuclear Cu2(COO)4 secondary building units with organic linkers to obtain a threedimensional NbO-type structure (Figure S4), which is isoreticular to the structure of NOTT-101.23 As depicted in Figure 1, both ZJU-40 and NOTT-101 have two different types

Recent studies have revealed that the immobilization of functional groups/sites, such as Lewis basic nitrogen sites, can not only significantly improve the C2H2 uptake but also differentiate C2H2 from other gas molecules.16−19 Furthermore, most of the NbO-type MOF-505 series exhibited exceptionally high C2H2 storage capacities attributed to their moderate surface areas, suitable pore sizes, and adjustable functionalities.20−22 These encouraging results may provide us a great opportunity to achieve both high gas selectivity and storage capacity in a single MOF by incorporating a high density of functional groups/sites into the NbO framework. With this goal in mind, we designed and synthesized a new tetracarboxylic acid ligand (H4L, Scheme 1) with a functional pyrazine group Scheme 1. Synthetic Route of the Organic Linker H4L

Figure 1. Portions of the single-crystal structures of NOTT-101 and ZJU-40 revealing functionalization of the nanocages within ZJU-40 through the incorporation of functional pyrazine groups. Green, red, blue, and black spheres represent copper, oxygen, nitrogen, and carbon atoms, respectively.

containing two Lewis basic nitrogen sites and reported the synthesis of its NbO-type MOF (ZJU-40) for C2H2 storage and separation. The activated ZJU-40a has a BET surface area and pore volume comparable to that of the isoreticular NOTT101a.23 Because of its high density of Lewis basic nitrogen sites and moderate pore sizes, ZJU-40a has the second highest C2H2 uptake capacity of 216 cm3 g−1 at ambient conditions for all of the reported MOFs so far, whose value is notably larger than that of NOTT-101a (184 cm3 g−1). However, the immobilized nitrogen sites show nearly no effect on the CO2 uptake at 1 bar and room temperature (87 cm3 g−1 for ZJU-40a vs 84 cm3 g−1 for NOTT-101a). As a result, ZJU-40a shows significantly enhanced selectivity for C2H2/CO2 separation (17−11.5) at room temperature compared with NOTT-101a (ca. 9), which is almost 2 times higher than that of HKUST-1 (10−5.8).15

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of nanocages: a small cage of about 10.2 Å diameter and a large irregular elongated cage of approximately 9.6 × 22.3 Å2. Because of the introduction of functional pyrazine groups, the cage surfaces of ZJU-40 are decorated with a large number of Lewis basic nitrogen sites, resulting in functionalized pores. These functionalized pores will certainly be favorable for gas adsorption and separation applications, especially for the storage of C2H2.16,18 Meanwhile, the inner cages of moderate size within ZJU-40 ensure that this material takes up a relatively high amount of C2H2.17,25 The combined feature of the functionalized pores and moderate inner cages within ZJU-40a encouraged us to evaluate its performance for C2H2 adsorption storage and the selective separation of C2H2/CO2 mixtures. As shown in Figure 2, the activated ZJU-40a absorbed a 675 cm3 g−1 amount of nitrogen at 77 K and 1 bar, and the nitrogen isotherm shows a significant type I sorption behavior without any hysteresis, characteristic of microporous materials. The BET surface area and pore volume of ZJU-40a were calculated

EXPERIMENTAL SECTION

Synthesis of the Organic Linker H4L. H4L was synthesized in two steps from 2,5-dibromopyrazine,24 as shown in Scheme 1. Details can be found in the Supporting Information. Synthesis of ZJU-40. ZJU-40 was synthesized by the solvothermal reaction of H4L with Cu(NO3)2·2.5H2O (see the Supporting Information for details).

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RESULTS AND DISCUSSION The organic ligand H4L was straightforwardly prepared through a Suzuki cross-coupling reaction followed by hydrolysis and acidification (Scheme 1). A solvothermal reaction of H4L with Cu(NO3)2·2.5H2O in a mixed solution of N,N-dimethylformamide (DMF)/acetonitrile/H2O (6:1:1, v/v) under acidic conditions at 80 °C for 1 day yielded blue block crystals of ZJU-40. ZJU-40 has a formula of [Cu2L(H2O)2]·7DMF·4H2O, derived from crystallographic data, thermogravimetric analysis (TGA), and elemental analysis. The high purity of the bulk material was further indicated by powder X-ray diffraction (PXRD), which matches well with the simulated one from Xray crystallographic data (Figure S2).

Figure 2. Nitrogen isotherm at 77 K with consistency and BET plots for the activated ZJU-40a sample. B

DOI: 10.1021/acs.inorgchem.6b00748 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry to be 2858 m2 g−1 and 1.06 cm3 g−1, respectively, which are comparable to those of the isoreticular NOTT-101a.26 The established high porosities and moderate pore sizes and the introduction of Lewis basic nitrogen sites in ZJU-40a have prompted us to examine its potential application for C2H2 storage. The single-component adsorption isotherms of C2H2 for ZJU-40a were collected up to 1 bar at 273 and 298 K, respectively (Figure S5). As shown in Figure 3a, the C2H2

areas, and OMSs, the significantly enhanced C2H2 uptake for ZJU-40a can probably be attributed to their different concentrations of Lewis basic nitrogen sites, where ZJU-40a has notably higher nitrogen concentrations (2.64 mmol cm−3) than ZJU-5a (1.28 mmol cm−3) and NOTT-101a (0 mmol cm−3). Because the C2H2 molecule has a linear form with two acidic hydrogen atoms at both ends, the incorporation of Lewis basic nitrogen sites in ZJU-40a might induce a specific affinity with C2H2 molecules in addition to the OMSs, presumably the H−CC−H···N (pyrazine) hydrogen bonding to further improve the C2H2 uptake. Such a phenomenon was also observed in other MOFs with uncoordinated nitrogen sites.16,18 This explanation can be further confirmed by the C2H2 adsorption enthalpies (Qst), where the value of Qst for ZJU40a is slightly higher than those for NOTT-101a and ZJU-5a over the whole C2H2 loading (Figure S6).18 Table 1 lists the comparison of ZJU-40a with some other promising MOFs for C2H2 storage at ambient conditions. Importantly, we can see that the C2H2 uptake (216 cm3 g−1) of ZJU-40a at 1 bar and 298 K is the highest so far for all of the reported MOFs except FIJ-H8,27 whose value is even significantly higher than those of two very famous MOFs with high densities of OMSs, known as HKUST-1 (201 cm3 g−1) and CoMOF-74 (197 cm3 g−1) (Table 1). Although CuTDPAT and UTSA-50a have nitrogen densities comparable with that of ZJU-40a, their C2H2 uptake capacities are still much lower than that of ZJU-40a (Table 1). This might be mainly attributable to the higher surface areas and more suitable pore sizes of ZJU-40a, which are also very important factors for achieving high C2H2 uptake capacity in MOFs.27 In addition, the storage density of adsorbed C2H2 in ZJU-40a is 0.18 g cm−3, which is only slightly lower than those in HKUST1, FIJ-H8, and CoMOF-74 but higher than most other promising MOFs (Table 1). When the storage temperature reduces to 273 K, the uptake and storage density of C2H2 for ZJU-40a can be further improved to 286 cm3 g−1 and 0.24 g cm−3 at 1 bar. Therefore, ZJU-40a is indeed a very promising candidate for C2H2 storage at ambient conditions. In contrast to the remarkably enhanced C2H2 uptake, as depicted in Figure 3, ZJU-40a takes up a similar amount of CO2 (87 cm3 g−1) at 1 bar and 298 K as NOTT-101a (84 cm3 g−1). This illustrates that the incorporation of Lewis basic nitrogen sites shows a negligible effect on the CO2 adsorption

Figure 3. (a) C2H2 and CO2 sorption isotherms of ZJU-40a (red) and NOTT-101a (blue) at 298 K, respectively. (b) IAST adsorption selectivities for an equimolar C2H2/CO2 gas mixture at 298 K.

uptake of ZJU-40a can reach up to 216 cm3 g−1 at 298 K and 1 bar. This capacity is really remarkable and is much higher than those of the isoreticular MOFs: NOTT-101a (184 cm3 g−1)20 and ZJU-5a (193 cm3 g−1).18 Considering that these three isoreticular MOFs have nearly the same structures, BET surface

Table 1. Comparison of Some Microporous MOFs for C2H2 Storage at 298 K and 1 bar MOF

SBET (m2 g−1)

C2H2 uptake (cm3 g−1)

densitya (g cm−3)

nitrogenb (mmol cm−3)

metalc (mmol cm−3)

Qstd (kJ mol−1)

ref

ZJU-40a FJI-H8 CoMOF-74 HKUST-1 ZJU-5a NOTT-101a Cu-TDPAT PCN-16 MgMOF-74 MOF-505 ZJU-25 NOTT-102a UTSA-50a

2858 2025 1018 1401 2823 2930 1938 2810 927 1139 2124 3342 604

216 224 197 201 193 184 178 176 184 148 175 146 91

0.18 0.23 0.27 0.21 0.13 0.15 0.16 0.15 0.19 0.16 0.13 0.10 0.13

2.64 0 0 0 1.28 0 2.93 0 0 0 0 0 2.62

2.64 3.13 7.25 4.36 2.56 2.61 2.93 3.03 7.15 4.11 2.02 1.94 2.62

34.5 32.0 50.1 34.0 35.8 32.4 30.8 34.5 34.0 24.7 25.4 22.0 39.4

this work 27 17 17 18 22a 19b 20 28 20 29 20 16

a Calculated density of adsorbed C2H2 in bulk material. bVolumetric concentrations of nitrogen sites in the framework. cVolumetric concentrations of OMSs in the framework. dThe C2H2 adsorption enthalpies at zero coverage.

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DOI: 10.1021/acs.inorgchem.6b00748 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Notes

for ZJU-40a, further confirmed by the similar CO2 adsorption enthalpies between ZJU-40a and NOTT-101a (Figure S7). Therefore, the introduction of Lewis basic nitrogen sites in ZJU-40a containing moderate pore sizes shows the preferential stronger interactions with C2H2 over CO2, leading to the ultrahigh C2H2 uptake and thus enhanced adsorption selectivity for the separation of C2H2/CO2. The exact reason why this MOF can enhance C2H2 gas uptakes more than CO2 ones is still not clear.30,31 We speculate that the position and orientation of the pyridine nitrogen sites within the ZJU-40a pores might prefer the binding of C2H2 over CO2. To investigate their performance on this important separation, we utilized the well-known Ideal Adsorbed Solution Theory (IAST) to calculate the adsorption selectivity of ZJU-40a and NOTT-101a for the binary C2H2/C2H4 mixtures (50:50, v/v). Figure 3b presents the IAST selectivity of the C2H2/CO2 separation for ZJU-40a and NOTT-101a at room temperature, respectively. It can be seen that ZJU-40a shows high adsorption selectivity in the range of 11.5−17 at 298 K, which is notably higher than that of NOTT-101a (8−9). It is worth noting that this selectivity value is also much higher than those of the most promising MOFs for this separation, such as HKUST-1 (5.8− 11)15 and UTSA-50a (5.5−15),16 indicating its bright promise for C2H2/CO2 separation at room temperature.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Welch Foundation (Grant AX1730 to B.C.), the National Natural Science Foundation of China (Grants 51272231 and 51229201), the Program for Innovative Research Team in University of Ministry of Education of China (Grant IRT13R54), and the Zhejiang Provincial Natural Science Foundation of China (Grant LZ15E020001).

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CONCLUSIONS In summary, we have designed and synthesized a new MOF (ZJU-40a) with a high density of Lewis basic nitrogen sites, exhibiting an exceptionally high C2H2 uptake of 216 cm3 g−1 at 298 K and 1 bar. This capacity not only is significantly higher than that of NOTT-101a but also sets the second highest material for C2H2 storage. Such an ultrahigh C2H2 uptake capacity was mainly attributed to the introduction of functional pyrazine groups with Lewis basic nitrogen sites into the pore surfaces and the suitable pore sizes. However, the immobilized Lewis basic nitrogen sites in ZJU-40a show a negligible effect on the CO2 storage capacity, leading to significantly enhanced C2H2/CO2 selectivity for ZJU-40a compared to that of NOTT101a, whose value is notably higher than those of the most promising MOFs, such as HKUST-1 and UTSA-50a. Our work here demonstrated that the incorporation of Lewis basic nitrogen sites combined with suitable pore sizes in MOF materials has the potential to not only improve the C2H2 storage capacity but also lead to high adsorption selectivity for the separation of C2H2/CO2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00748. Crystallographic data, NMR, TGA, PXRD, heats of adsorption of C2H2, C2H2 sorption isotherms, and additional figures (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

These authors contributed equally. D

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b00748 Inorg. Chem. XXXX, XXX, XXX−XXX